It is necessary to provide a non-sticking interface between a nano-structured master and the polymer to be embossed in the replication of nano-structures by imprinting processes, such as hot embossing. This allows demolding without degrading the replication fidelity. Adhesion during the mold release may cause damage to the replica. Further, residual polymer structures may contaminate the surface of the mold. A prerequisite for a successful imprint process is that the mold used should be chemically and mechanically stable and should also adhere poorly to polymers such that sticking to the mold is avoided.
During the last years a lot of effort has been made to minimize the adhesion forces at the stamp/polymer interface in nano-imprint lithography [1-3] mainly with the establishment of thin fluorinated films deposited onto the stamp surfaces [4-7]. These anti-sticking films lead to the reduction of surface energy of the stamps and improve the release of the stamp from the imprinted polymer. It is well known that for gold surfaces, the adsorption of fluorinated alkyl thiol self-assembled monolayer (SAM) films leads to both highly hydrophobic and lipophobic surfaces demonstrating contact angles with water of 120° and with hexadecane of 81° [8]. U.S. Pat. Nos. 5,512,131 and 6,380,151 disclose the use of SAM forming species as part of a method of patterning a material surface for use in microelectronic and optoelectronic applications. Neither of these patents however, utilise the SAM films for the purpose of reduction of adhesive forces.
When addressing the reduction of adhesive forces, two alternative solutions presently show promise. The first alternative involves the deposition of polytetrafluoroethylene (PTFE) films onto structured stamps with the help of a CF4/H2 or CHF3 plasma [4]. In this case, the polymer is only bound weakly to the substrate and the film has to be renewed after a few imprints in order to maintain the imprint quality. The second alternative involves the formation of anti-sticking films of fluorinated alkyl silanes using chemical vapour deposition possessing film thicknesses in the monomolecular region [5-7]. The direct chemical bond of the silane groups, especially for silicon stamps, with the substrate surfaces results in good imprint results of patterns with feature sizes down to 20 nm [5].
From the literature it is also known that alkyl phosphate salts, alkyl phosphoric acids, and alkyl phosphonic acids produce well-ordered self-assembling monolayers on oxidized surfaces of aluminium, tantalum, niobium, zirconium, and titanium taking advantage the strong ionic linkage between the phosphate group and the oxidized metal surface [9-13]. After heating of the samples, the films demonstrate a strong resistance against mechanical force as well as chemical stability against solvents [14]. Also fluorinated alkyl phosphoric acid derivates have been recently deposited onto oxidized aluminium and alumina surfaces [15,16].
With regards to substrates, the application of polycrystalline nickel sheets instead of silicon wafers as stamp materials may be regarded as an industrially more relevant alternative. Nickel (Ni) stamps are commonly used in several industrial applications of nano-imprint lithography (NIL).
Ni especially may be regarded as a highly suitable material for the preparation of phosphoric acid based surfactants in an industrially relevant process. The ionic nature of Ni-oxide increases the possibility of an ionic linkage between the phosphate group and the Ni stamp surface without the application of additional adhesion promoters, allowing imprints of patterns with feature sizes down to 100 nm. The ionic nature of the Ni-oxide surface is thus considered suitable for the application of acid-based surfactants or surface modification agents while discouraging the use of silane agents, which in their turn, are more applicable when silicon stamps or silicon substrate surfaces are used [5, 17-19].
WO 2004/000567 A1 discloses anti-sticking films deposited on Ni stamps, used in nanoimprint lithography. Phosphate groups are among others used as linkage groups to metal films (Ti, Zr, Nb, Ta, Al and a mixture thereof) deposited as adhesion promoters onto Ni stamps (claims 3, 2, 10).
Problem
Different performed imprint tests and spectroscopic investigations have demonstrated that anti-sticking layers adsorbed on Ni stamps that are composed of alkyl esters of phosphoric acids or phosphates—having an O bridge in-between the phosphorous atom and the alkyl chain as in FIGS. 1A and 1B—are not optimum to withstand an industrial imprint process of some thousands of imprints. Moreover, storage of the stamps—i.e. the exposure of Ni stamps with air humidity for a long time—will additionally reduce the lifetime of those stamps. The reason for this is mainly that the heating-cooling cycles in air humidity will continuously reduce the quality of the anti-sticking film. This behavior is strongly caused by modifications within the Ni oxide/hydroxide surface layer. The oxidized/hydroxidized Ni surface layer is a highly complex system in case of both corrosion and catalytic activities [26,27,28,29]. The layer is predominantly composed by NiO or Ni2+, Ni2O3 or Ni3+, Ni(OH)2 and NiO(OH) [26,27], partially weakly bound to the Ni surface. NiO is the most common and stable Ni oxide whereas Ni2O3 are often called as lattice defects within the NiO lattice. These lattice defects can be regarded as the origin of catalytic reactions of nickel oxide [26,27]. NiO(OH) can be considered as an unstable intermediate within a reaction Ni(OH)2→NiO under desorption of hydroxides [26].
In particular, heating of the Ni stamps will initiate the desorption of OH or O or H containing species/radicals from the oxidized/hydroxidized Ni surface, having the capacity to break bonds within the phosphate/phosphoric acid based anti-sticking molecule. Here, species means: atoms, molecules and/or dissociatively generated clusters of atoms which can be charged or neutral. Cooling in air humidity will lead to the growth of a new oxide/hydroxide layer due to the chemisorption of H2O and oxygen from the air (see discussion below). Different examples of heterogeneous catalysis of oxidized Ni with H2O producing highly reactive gases are well known from the literature [26]. H2O exposure—i.e. storage in air humidity—in connection with surface lattice defects results in similar reactions as described above [26,27]. Therefore it is most probable that chemical modifications of the oxidized Ni stamp substrate are the reasons for the problems of the reliability and durability of anti-sticking films comprising phosphates or phosphoric acids.
In the course of our X-ray photoelectron spectroscopy (XPS) investigation desorptions and damages of F8H2—PS films adsorbed on oxidized Ni surfaces could be observed after thermal annealing at already relative low temperatures in UHV (150-200° C.). F8H2—PS: Heptadecafluoro(1H,1H,2H,2H-tetrahydro)decyl-monophosphate (CF3—(CF2)7—CH2)2—O—P(O)—O22− 2+2NH4). In particular, after heating to 150° C. some amount of the F8H2—PS film has been desorbed from the surface, whereas the chemical composition of the fluorinated alkyl phosphate film does not change significantly. The removal of some amounts of the anti-sticking film is most probably related to the conversion from Ni2O3 and/or Ni hydroxides to NiO of the oxidized/hydroxidized Ni surface that has been observed at the same temperature. Strong damages within the fluorinated alkyl phosphate molecules occur after heating to 200° C. At that temperature 70% of fluorinated alkyl chains are removed from the surface. The most of the anti-sticking film is certainly destroyed. The growth of the C—O, C—H and F containing species on the stamp surface at 200° C. can be interpreted as a dissociation of fluorine from the CF-chains according to the formula: CF3→CF2→CF→C (such reactions are known in photo-dissociation of fluorinated alkyl silanes on Ti when the molecules are exposed to weak X-ray light [28]).
In recapitulation of the XPS investigation it can be concluded that due to heat treatment, bonds within the molecule could be destroyed quite easily, whereas the ionic P—O—Ni linkage between the F8H2—PS molecule and the Ni substrate surface has been observed to be quite stable. This behaviour is strongly correlated to properties and modifications of the Ni oxide/hydroxide layer, because F8H2—PS molecules—not adsorbed on a surface—demonstrate thermal stabilities up to 230° C. At that temperature the molecules could be sublimated in vacuum without observed modification within the molecular structure investigated by NMR (nuclear magnetic resonance).
It is most probable that the reduced decomposition temperature of the F8H2—PS films adsorbed on oxidized Ni is particularly caused by OH− radicals or H2O2 molecules or other OH containing species removing from the surface at low temperatures followed by the desorption of oxygen at higher temperatures [26]. These species will react with moieties of the F8H2—PS molecules and destroy the molecules. However, the exposure with air humidity will regenerate the oxidized/hydroxide Ni surface due to the adsorption of H2O and oxygen from the air environment, so that heating-cooling-cycles will create a continuative decomposition process. The XPS measurements demonstrate that the F8H2—PS film is damaged in a similar way as already observed in X-ray beam induced damages of fluorinated alkyl silanes adsorbed on oxidized Ti surfaces due to photo-generated electrons escaping from the oxidized metal [28]. So it can be assumed that the adsorbed F8H2—PS molecules can maybe be damaged by charged radicals or highly reactive molecules escaping from the oxidized Ni surface in a similar way as observed for adsorbed fluorinated alkyl silanes exposed by photo-generated electrons.